Paint compositions comprising hydroxyl group containing acrylic resins and crosslinking agents which have functional groups which react with hydroxyl groups are widely used in the automobile painting field. Conventionally, copolymers of radically polymerizable monomers which have hydroxyl groups and other copolymerizable monomers have been used in the main for the hydroxyl group containing acrylic resins which are used in these paint compositions, but more recently a demand has arisen in the automobile painting field for paints, and especially clear paints, where the paint film has acid rain resistance and scratch resistance, and the development of paints which form paint films which have excellent scratch resistance and durability has become a pressing need.
Paint compositions which have as essential components (A) an acrylic copolymer of acid value from 25 to 125 mgKOH/g and hydroxyl group value of from 30 to 150 mgKOH/g which includes (meth)acrylic acid derivative monomer units and (B) an acrylic copolymer of acid value from 230 to 1500 and hydroxyl group value from 30 to 150 mgKOH/g which includes unsaturated monomer units which have epoxy groups, where the components (A) and (B) are copolymers which may include unsaturated monomers which are ε-caprolactone modified forms of acrylic monomers which have hydroxyl groups, and, moreover, (C) amino resin are known as a method of obtaining paint films which have excellent acid resistance and scratch resistance (for example, see Japanese Unexamined Patent Application Laid Open H5-171103). However, with these paint compositions there is a weakness in that it is difficult to provide a paint film with both scratch resistance and staining resistance.
Furthermore, high solid fraction paint compositions which contain (A) from 5 to 30 wt % of hydroxyl group containing lactone modified oligomer of weight average molecular weight not more than 1000 and hydroxyl group value from 200 to 800 mgKOH/g, (B) from 5 to 50 wt % hydroxyl group containing resin of weight average molecular weight from 1000 to 6000 and hydroxyl group value from 50 to 200 mgKOH/g, (C) from 30 to 70 wt % of polyisocyanate compound and (D) from 3 to 30 wt % melamine resin are known as paint compositions which have a high solid fraction and of which the finished appearance of the paint film, paint film hardness and the acid rain resistance and scratch resistance of the paint film are excellent, using two types of hydroxyl group containing resin in the same way (for example, see Japanese Unexamined Patent Application Laid Open 2002-105397). However, with these paint compositions there is a weakness in that the staining resistance of the paint film is inadequate.
Furthermore, paint compositions which have as essential components (a) from 30 to 90 parts by mass of a lactone modified acrylic polyol resin which has been obtained by the ring-opening addition reaction in the absence of a catalyst on adding from 10 to 200 parts by mass of lactone compound to 100 parts by mass of an acrylic polyol resin of which the hydroxyl group value is from 75 to 250 mgKOH/g and the acid value is from 0.5 to 50 mgKOH/g, (b) from 10 to 70 parts by mass of polyisocyanate compound and (c) from 0.01 to 20 parts by mass of a specified alkoxysilane partially hydrolyzed condensate are known as paint compositions where the staining resistance and impact resistance of the paint film are excellent and where at the same time the external appearance, weather resistance, water resistance and the like are also excellent (for example, see Japanese Unexamined Patent Application Laid Open 2003-313493). However, these paint compositions have a weakness in that the re-coating adhesion properties are inadequate.
Furthermore, a resin composition for painting purposes which contains a starburst-type polyester oligomer of number average molecular weight from 1,500 to 15,000 of which the aromatic ring content is from 0.9 to 3.3 rings/kg of resin, the lactone content is from 20 to 60 wt %, the hydroxyl group value is from 56 to 300 mgKOH/g and which has a plurality of radial branches from a supposed central molecule with a further plurality of branches from each branch, and a hardening agent selected from among the group comprising aminoplast resin, isocyanate pre-polymer and blocked isocyanate pre-polymer are known as paint compositions where the paint film is provided with flexibility (for example, see Japanese Unexamined Patent Application Laid Open H9-157589). However, these paint compositions have a weakness in that the weather resistance of the paint film is inadequate when they are used as top-coat paints on automobiles.
Furthermore, a paint composition which contains a hydroxyl group containing acrylic resin (A) of which the hydroxyl group value is from 80 to 160 mgKOH/g which has been obtained by the radical copolymerization of (a) from 8 to 30 wt % of primary hydroxyl group containing monomer selected from among 4-hydroxybutyl(meth)acrylate and the ε-caprolactone modified vinyl monomers obtained by bringing about the ring-opening polymerization of ε-caprolactone with a hydroxyalkyl(meth)acrylate), (b) from 10 to 40 wt % of a secondary hydroxyl group containing monomer and (c) from 30 to 82 wt % of other polymerizable unsaturated monomer, and a hardening agent (B), and which contains from 60 to 90 wt % of the acrylic resin (A) and from 10 to 40 wt % of the hardening agent (B) on the basis of the total solid fraction of the acrylic resin (A) and the hardening agent (B) is known as a clear paint composition which has excellent low temperature hardening properties, paint film adhesion and finish for example (for example, see Japanese Unexamined Patent Application Laid Open 2004-314060). However these paint compositions have a weakness in that the scratch resistance of the paint film is inadequate.